Extracellular sodium concentration is important in regulating blood volume and consequently, arterial pressure. There are a variety of lines of evidence implicating the central nervous system (CNS) in the control of renal sodium excretion and cardiovascular regulation. Recently the periventricular tissue surrounding the anteroventral third ventricle (AV3V) has been implicated in several aspects of electrolyte homeostasis and vascular control. Ablation of AV3V tissue affects thirst, antidiuretic mechanisms, sodium metabolism, pressor responses, and development of several forms of experimental hypertension. Recent studies have indicated that AV3V lesions attenuate the natriuretic response following volume expansion which is important in restoring blood volume to normal levels. The present research is directed at characterizing the role of AV3V tissue in overall body-sodium balance. In addition, the proposed research is designed to determine the critical tissue within the AV3V region, and neural pathways involved in blood-volume regulation through changes in renal sodium excretion. This analysis will measure blood and urine composition during several experimental procedures which alter renal sodium excretion in animals which have undergone various treatments effecting AV3V periventricular tissue or associated pathways, including electrolytic and mechanical ablation and discrete knife cuts. The proposed research will lead to insights regarding CNS regulation of peripheral blood pressure, and the role of the AV3V region and associated brain area in sodium balance and several forms of experimental hypertension.